1. Field of the Invention
The present invention relates to exhaust gas purification filters and methods of producing exhaust gas purification filters, capable of purifying exhaust gas emitted from internal combustion engines, in particular, gasoline direct injection engines.
2. Description of the Related Art
It is generally known for exhaust gas emitted from a diesel engine to contain particulate matter (PM) such as carbon particles, etc. Such PM causes air pollution which is the introduction of particulates, biological molecules, or other harmful materials into the Earth's atmosphere, possibly causing disease or death to humans, damage to other living organisms such as food crops, or to the natural or built environment. In order to regulate an amount of the discharge of harmful industrial waste such as PM, each country has air pollution control laws.
By the way, diesel automobiles of diesel engines are equipped with an exhaust gas purification filter in order to eliminate PM from exhaust gas emitted from diesel engines. That is, the exhaust gas purification filters filter and collect PM contained in exhaust gas. For example, a patent document, Japanese patent laid open publication No. H07-332064 has disclosed an exhaust gas purification filter comprised of a honeycomb structural body. This honeycomb structural body has a plurality of cells and partition walls. The cells are surrounded by the partition walls and formed along a longitudinal direction of the honeycomb structural body. An opening of one end of each of the cells is plugged by a plug member.
Recently, because the vehicle emissions control of reducing motor vehicle emissions is becoming stricter year by year in view of environmental protection, there is a strong demand to decrease PM such as carbon contained in exhaust gas emitted from gasoline direct injection engines of gasoline vehicles in addition to diesel engines of diesel vehicles and more improve a fuel efficiency of vehicles. In particular, it has been pointed out that gasoline direct injection engines emit PM which is smaller in particle size than that emitted from diesel engines.
In general, diesel engines emit exhaust gas which contains PM having an average particle size within a range of 60 to 100 nm. On the other hand, gasoline direct injection engines emit exhaust gas which contains PM having an average particle size within a range of 40 to 80 nm. Accordingly, because there is a possible emission control in the future on the basis of the number of PM particles in addition to an amount of PM, it has been strongly considered that gasoline vehicles should be equipped with an exhaust gas purification filter in order to filter PM from exhaust gas.
However, in general, exhaust gas emitted from gasoline engines such as gasoline direct injection engines has a high temperature which is higher than a temperature of exhaust gas emitted from diesel engines. For example, diesel engines emit exhaust gas having a temperature within a range of 150 to 400° C. On the other hand, gasoline direct injection engines emit exhaust gas having a temperature within a range of 350 to 700° C. For this reason, exhaust gas emitted from the gasoline direct injection engines has an expanded volume more than a volume of exhaust gas emitted from diesel engines. When gasoline vehicles use an exhaust gas purification filter for use in diesel vehicles in order to filter and collect PM, a pressure loss of the exhaust gas purification filter increases. This decreases an output torque of the engine. Accordingly, there is a strong demand to adequately collect PM while suppressing increasing of the pressure loss of the exhaust gas purification filter.